Ultrasonic investigation of rails is well known. For example U.S. Pat. No. 4,165,648 describes a wheel assembly, which is adapted to ride on top of a rail and contains a plurality of ultrasonic transducers for investigating different parts of a rail. A pair of transducers are oriented to investigate the rail along opposite longitudinal directions, another transducer is oriented to vertically investigate the web of the rail and a fourth transducer is intended as a side-looker with which side portions of the rail head are to be laterally investigated.
The side-looker is mounted on a yoke assembly, near the axis and is positioned off to one side to investigate an opposite side of the rail head. The transducer produces a beam intended to have a 40 degree angle relative to the normal to the rail surface inside the rail head so that the beam will intersect one of the lower corners of the head. Although this transducer can be effective for detecting vertically split rail heads it does not accommodate the effect of the curvature of the rail head and the head wear that one encounters under field conditions.
The curvature of the rail head causes a variety of angles of both compressional and shear components to be present inside the rail head and thus creates difficulties in interpreting the resulting signals detected at the transducer. For example, if the resultant angle is to be 40 degrees inside the head for a compressional wave, then a 20 degree shear wave will also be present. Since a mode conversion is relied upon to accomplish the resultant angle in the rail head, there is but a limited range of incident angles available inside the wheel. This limitation arises from the use of essentially a common exit point for all of the transducers mounted inside the wheel and the resulting need to avoid interference with the other transducers by internal wheel reflections from the side-looking transducer.
The '648 patent illustrates that for one particular beam direction, the corners of one rail head and its proportionately smaller sizes are likely to be intersected by a 40 degree beam. However, when the rail head undergoes significant wear on its upper surface, as is often the case, the beam will miss the lower corner. Instead the beam is likely to strike an inner surface near the web, which scatters the beam inside the head, and fails to provide the type of return needed to detect vertical flaws near the side of the rail head.